Two tick-borne ehrlichioses, human monocytic ehrlichiosis (HME) caused by Ehrlichia chaffeensis and human granulocytic ehrlichiosis (HGE) caused by Ehrlichia equi or a closely related organism, have emerged in the United States since 1986. The long-range goal of this research is to enhance the diagnosis, prevention, and control of human ehrlichioses by determining their geographic distributions, epidemiologic risk factors, and genetic relationships. Deer are intimately linked to HME and HGE because they serve as reservoirs, key hosts for tick vectors, or both. Deer harbor a third potentially zoonotic but un-described Ehrlichia. The rationale for the proposed research is to exploit the unique role that deer play in the natural history of these ehrlichiae to accomplish a major objective of elucidating the landscape and molecular epidemiology of these pathogens. To accomplish this objective, we will pursue five specific aims: (1) develop and implement a surveillance system utilizing deer as natural sentinels to delineate the geographic distributions of E. chaffeensis, the HGE agent, and the deer Ehrlichia organism;(2) experimentally evaluate the suitability of deer as a reservoir for the HGE agent;(3)isolate and characterize the deer Ehrlichia organism in order to facilitate development of diagnostic assays required to test humans for infection by this agent;(4)determine the molecular variation of strains of E. chaffeensis and the HGE agent from deer populations and compare this genetic variability with isolates from humans; and,(5)utilize geographic information system(GIS)technology and spatial analyses to relate the distributions of E. chaffeensis, the HGE agent, and the deer Ehrlichia to ecological variables and compare these landscape epidemiologic patterns with the reported distributions of HME and HGE cases. When completed this research should:(1)provide a better estimate of the relative risk of infection in different areas;(2)identify areas where increased diagnostic or preventive efforts are needed;(3) provide data useful for designing more powerful epidemiologic or risk assessments; and, (4) provide biologic material for improved diagnostic assays.